<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // typeof 判断数据类型
    // console.log( typeof 123 )  // number
    // console.log( typeof 'qwer' )  // string
    // console.log( typeof '123' )  // string
    // console.log( typeof true )  // boolean
    // console.log( typeof null )  // object
    // console.log( typeof undefined )  // undefined

    // var test
    // console.log( typeof test )  // undefined
  </script>
</body>
</html>